Abstract

The Seventy Mile Range Group is a Cambro-Ordovician, dominantly submarine volcano sedimentarysuccession that occurs within the Mount Windsor Subprovince of northernQueensland, Australia. Detailed facies analysis of the Mount Windsor Formation andTrooper Creek Formation, between Coronation homestead and Trooper Creek prospect(approximately 15 km strike length), has clarified the facies architecture in this part of theSeventy Mile Range Group. The rocks have been affected by regional greenschist faciesmetamorphism and deformation, and hydrothermal alteration is intense around VHMSdeposits.The Mount Windsor Formation comprises associations of rhyolitic to dacitic autoclasticbreccia and coherent facies, 100-500 m thick, which form submarine lavas, domes andsyn-volcanic intrusions. The Mount Windsor Formation (300-3500 m thick) isconformably overlain by the Trooper Creek Formation (500-4000 m thick). The TrooperCreek Formation is divided into two members, the Kitchenrock Hill Member and theoverlying Highway Member. The new stratigraphic scheme is based on mappablecompositional and lithological variations which reflect changing provenance. TheKitchenrock Hill Member (90-110 m thick) includes rhyolitic to dacitic syn- and posteruptivevolcaniclastic facies, minor syn-sedimentary intrusions and rare siltstone units.Some volcaniclastic units contain rounded clasts with geochemical and petrographicproperties which suggest they were sourced from the Mount Windsor Formation.Rounded clasts were reworked prior to deposition and imply that the source areas weresubaerial or shallow marine. The Highway Member comprises compositionally andtexturally diverse volcano-sedimentary facies, including rhyolitic to basaltic lavas andintrusions, andesitic scoria- and bomb-rich breccia, dacitic to rhyolitic pumice breccia,and volcanic and non-volcanic sandstone and siltstone.The presence of turbidites, hyaloclastite and fossils within the Kitchenrock Hill Memberand Highway Member suggest that the depositional setting for the Trooper CreekFormation was largely submarine and below storm wave base. The exception is in theupper part of the Highway Member at Trooper Creek prospect, where microbialites,gypsum molds, and traction current structures indicative of wave activity, collectivelysuggest a depositional environment above stonn wave base. The lithofacies exposed atTrooper Creek prospect suggest that shoaling of the succession occurred in response toconstruction of a small, submarine andesitic volcano which temporarily emerged abovesea level. Growth of the edifice involved: (I) eruption of lava and intrusion of synsedimentarysills; (2) strombolian-style volcanism in a near-stonn-wave-baseenvironment; (3) hydrovolcanic interactions above storm wave base and possiblysubaerially; and (4) post-eruptive and possibly syn-eruptive degradation of the volcanicedifice. When subsidence due to compaction and/or tectonism outpaced accumulation, thedepositional environment returned to below storm wave base.Syn-eruptive pumiceous and crystal-rich sediment gravity flow deposits in the TrooperCreek Formation were sourced from rhyolitic to dacitic eruptions at volcanic centreswhich are either; located outside the study area, not exposed, or not preserved. Theabundance of pyroclasts (principally pumice, shards and crystals) reflects the importanceof explosive magmatic and/or phreatomagmatic eruptions, and suggests that the sourcevents were in shallow water or subaerial settings. Some key units may be traceable forover 30 km, and are an important framework for exploration within the Trooper CreekFormation.The Highway Member includes the products of intrabasinal, non-explosive silicic tointermediate eruptions that formed lava- and intrusion-dominated volcanic centres. Thevolcanic centres are: (1) dominated by syn-sedimentary sills and cryptodomes; (2)comprise thick (> 5 km) lava complexes; or (3) form lava-lobe hyaloclastite domes, up to500 m thick and at least 1.5 km long. The Cu-Au-rich Highway-Reward massive sulfidedeposit is hosted by one small syn-sedimentary intrusion-dominated volcanic centre.Detailed mapping of contact relationships and phenocryst populations suggest thepresence of more than thitteen distinct porphyritic units in a volume of I x I x 0.5 km.The peperitic upper margins of more than 75% of these units suggests that they wereemplaced as syn-sedimentary sills and cryptodomes. Evidence for partial extrusion of lavais limited to one rhyolite. The shape, distribution and emplacement mechanisms of theunits were influenced by: (1) the density of magma relative to wet sediment; (2) thelocation and shape of previously emplaced lavas and intrusions; and (3) possibly synvolcanicfaults which may have acted as conduits for rising magma. Deformation anddisruption of bedding, resedimentation, dewatering and Iow-grade metamorphism of theenclosing sediment accompanied emplacement of the intrusions. The resulting patterns ofpermeability and porosity in the volcanic succession are inferred to have stronglyinfluenced the location and evolution of the syn-genetic hydrothermal system whichformed the Highway-Reward massive sulfide deposit. The pyrite-chalcopyrite pipes andmarginal pyrite-sphalerite-galena-barite mineralisation are largely syn-genetic sub-seafloorreplacements of the host sediment, cryptodomes and volcaniclastic deposits.Hematite alteration and ironstones are regionally distributed and are not preferentiallyassociated with mineralisation. Most ironstones have geochemical and lithofaciescharacteristics which suggest they are the deposits from low temperature fluids circulatingaround lavas and intrusions and within the proximal facies association of shallowsubmarine volcanoes. At Handcuff, some ironstone lenses have REE and trace elementsignatures which suggest they may be associated with as yet undiscovered massivesulfide mineralisation. Many ironstones are sub-seafloor replacements of pumiceousbreccia and sandstone beds.The top of the Highway Member marked the end of intrabasinal volcanism and volcanicdominatedsedimentation. This was then followed by a phase of post-eruption erosion ofthe subaerial to submarine source areas that led to an influx of variably rounded volcanic andbasement-derived detritus into the submarine basin, during deposition of theoverlying Rollston Range Formation.This research provides insights into understanding comparable modern and ancientsubmarine volcanic successions and assessing prospective host sequences for massivesulfide mineralisation.

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